Server/Client connections aren't always closed at the end of each unit test

[gabis-precog]

As a result of connections not always closing, asyncio tasks are not ended properly during tests.

Steps to reproduce

• in "def cancel_if_task_exists" function, set logger().warning('Runtime error', exc_info=True) to error level
• in setup.cfg enable treating warnings as errors (comment out the relevant settings)

Expected: no failing tests

Actual : tests randomly fail with various errors caused by race conditions on test teardown.

This is likely specific to the quic transport, although might not be.